The broad concept of utilizing controlled environments to promote storage life of perishable commodities such as fruits and grains is well known. Though the use of refrigeration units has been a common technique of preserving perishable goods, certain types of controlled atmosphere systems are currently known which operate through the controlled manipulation of carbon dioxide and oxygen levels within transport/shipping containers in addition to the use of refrigeration. In this respect, hollow fiber permeable membranes are often used for air separation in maintaining controlled carbon dioxide and oxygen levels within grain elevators and controlled atmosphere warehouses. As with most controlled atmosphere systems, those utilizing permeable membranes require a reliable source of compressed air.
Recent advances in membrane technology have increased the efficiency and decreased the size of gas-permeable membrane systems thereby making the application of membrane technology more feasible for controlled atmosphere transport applications. However, though the technology associated with permeable membranes has advanced, transport refrigeration units typically do not include controlled atmosphere devices because of the reduced cargo space, increased weight, power and cost. Additionally, the corrosive marine environments and extreme temperature parameters typically encountered by mobile containers makes controlled atmosphere applications in conjunction with such containers very difficult since the compressor used to supply compressed air to such controlled atmosphere system oftentimes disfunctions when subjected to such conditions.
A device for producing a controlled atmosphere which utilizes permeable membranes is disclosed in U.S. Pat. No. 4,187,391 to ROE. The ROE reference discloses a method and apparatus for producing a controlled atmosphere within a stationary storage container, such as a grain elevator or produce warehouse, wherein air from the interior of the controlled atmosphere warehouse is recirculated via a membrane system to reduce its oxygen and carbon dioxide content. The ROE system however, due to a recirculation of air within the interior of the container, creates a vacuum within the container and thus appears to promote leakage of ambient air into the interior of the container. Additionally, the ROE patent utilizes a compressor located exterior of the controlled environment container as well as a conventional air cleaner to eliminate oil and/or other contaminants from the controlled air stream.
Most controlled atmosphere systems, including that as disclosed in the ROE reference, utilize oil-flooded rotary screw or rotary vane air compressors which are typically mounted to the exterior of the transport container. However, controlled atmosphere systems incorporating exteriorally mounted, oil-flooded compressors possess certain inherent deficiencies which detract from their overall utility. One such deficiency which pertains to permeable membrane-controlled atmosphere systems relates to oil carryover. Since trace amounts of oil eventually reduce membrane performance, it is critical that oil be prevented from entering the permeable membranes. However, all oil-type compressors pass small amounts of oil into the compressed air and therefore into the membranes. Though many of these systems are provided with oil filters, removing all trace amounts of oil by replaceable filters is very difficult and hence reduction in system performance results. Thus, in the actual operating environment, the filters are typically not changed on the regular intervals necessary to assure adequate performance and oil removal.
A second deficiency relates to temperature variation. As can be appreciated, ocean shipping containers travel all over the world and therefore encounter typical operating temperature parameters between minus 50-degrees Fahrenheit to 130-degrees Fahrenheit. Most standard air compressors used in conjunction with controlled atmosphere systems have temperature operating parameters of 32-degrees Fahrenheit to 110-degrees Fahrenheit without special changes to the compressor itself or to the oil with which it is utilized. Since the compressor may see extreme temperatures on opposite ends of the temperature scale during a single voyage, the compressor must be adapted to operate continuously throughout the temperature range without oil changes or other servicing.
A further deficiency relates to corrosion. In this regard, any compressor mounted on the exterior of the ocean container will be exposed to salt water corrosion, rain, sleet and snow. Thus, in configurations such as that previously discussed with respect to the ROE reference wherein the compressor is mounted to the exterior of the container, the compressor would necessarily be subject to the aforementioned conditions. The present invention specifically overcomes these and other deficiencies associated with prior art controlled atmosphere systems.